Indirectly heated cathode structure for magnetrons



Nov. 19, 1957 A. J. YOUNG E L 2,813,994

INDIRECTLY HEATED CATHODE STRUCTURE FOR MAGNETRONS Filed 00" 31, 1955 H511, PRIOR ART ATTORNEY- INDIRECTLY HEATED CATHODE STRUCTURE FOR MAGNETRONS Application October 31, 1955, Serial No. 543,974

Claims priority, application Great Britain September 12, 1955 Claims. (Cl. 313-340) This invention relates to indirectly heated cathode structures for magnetrons and more particular to such cathode structures of the kind in which the cathode heater is within a cathode-carrying tubular or similar box which is, when the magnetron isin use, so positioned as to be subjected to high frequency fields.

The invention is illustrated in, and explained in connection with, the accompanying drawings in which Figure 1 is a schematic sectional elevation showing a typical known form of magnetron cathode structure; Figure 2 is a view similar to Figure 1 of one embodiment of the present invention; and Figure 3 is a part view of a modification and Figure 4 is a part of another modification.

In order that the invention may be understood the typical known magnetron cathode structure shown in Figure 1 will first be described together with a practical defect found to he possessed by that structure.

Referring to Figure 1 this shows the indirectly heated cathode structure of a multiple cavity magnetron having the usual anode block with the cathode at the axis thereof. The anode block is only partly represented at 1. Insulated from the anode block and carried thereby so as to extend radially inwards are two metal carrier rods 2 and 3 whose attachment to the anode block are not shown in the figure. These carrier rods also form external connections to the cathode heater as will be seen later. Carried by the carrier rods 2 and 3 are two metal discs 4 and 5 of which the former forms one closure and piece of a metal tube 6 on the surface of which is electron emissive material (not separately shown) so that the tube 6 forms the cathode proper. At the end of the tube 6 remote from the disc 4 is an insulating locator piece 7 which is attached to the disc 5 by means of a metal cap 8 holding the said insulator 7 and welded to the said disc 5. The cathode heater is in the form of a helical winding 9 concentrically within the box structure constituted by the parts 4 to 8 which is connected to the discs 4 and 5 at the points 10 and 11. The heater is of wire coated with alumina or some similar material, the purpose of which is to insulate the heater turns from one another and also from the interior of the cathode tube 6.

It will readily be appreciated that if, in a construction as shown in Figure 1, the heater is so dimensioned that it is possible for it to touch the cathode tube 6, breakdown of the insulation may occur at any point of contact and the heater will fail. Even, however, if the heater diameter is made such as to give a good clearance from the inside of the tube 6 so that there is no risk of contact between heater and tube, the operating life of the heater is found still to be undesirably short, breakdown of the heater commonly occuring after a comparatively short period of use somewhere in those end turns of the heater near the insulator 7. Careful experiment supports the belief that this trouble is caused by the penetration of high frequency fields through the gap caused by the 2,813,994 Patented Nov. 19, 1957 presence of the' insulator 7 in what would otherwise be a completely enclosing metallic box. While there is still some doubt as'to' whether this theory of the cause of breakdown is correct and sufiicient, it has served as a basis of thought which has led to the present invention which has been found in experimental practice to eliminate the defect and to increase the life of the heater very substantially indeed as compared to that of an otherwise comparable known construction.

According to this invention a magnetron cathode structure wherein a cathode heater is contained within a cathode-carrying box-like housing which is partially conductive but has a metallic end piece and an insulator insulating said end piece from the remaining metal parts of the housing is provided with means for providing additional capacity between said end piece and said remaining metal parts of said housing.

The means for providing the additional capacity may take any of a variety of diiferent forms. For example, in one form of construction the end turns of the heater adjacent an end insulator forming part of the housing is encircled by a metallic tube electrically and mechanically united with a metallic end plate of the housing to which said end of the heater is connected and which is insulated from the rest of the housing. In another form of construction, a skirt member is electrically and mechanically connected to a cathode tube forming part of the housing near an end which incorporates an insulator, said skirt member extending over the insulator towards a metallic end plate of the housing on the other side of said insulator and to which the adjacent end of the heater is connected. In a third form of construction a metal disc-like member apertured to pass the connection from the heater to the metallic end plate adjacent the end insulator, is placed over said insulator on the side thereof remote from said metallic end plate and is connected to the cathode tube.

Although the invention is not limited to the production of any particular value of additional capacity it is found in practice that the production of an additional capacity of the order of 5 to 10 pico-farads is satisfactory. In a known arrangement as shown in the accompanying Figure l the inherent self capacity between the cathode tube 6 and the metallic end plate 5 of the housing is only of the order of 2 to 3 pico farads or thereabouts so that, in practice the invention involves more than doubling the capacity normally present at the point in question.

It would appear that the invention operates by virtue of the fact that the increased capacity reduces the effective heater impedance for high frequency over at least the vulnerable length thereof but whether or not this theory of operation is correct and sufficient experiment has shown that a substantial practical advantage is obtained by the use of the invention.

Since like references are used for like parts throughout the figures the embodiments of the invention show in Figures 2, 3 and 4 require little additional description and it is thought that these figures will be to a large extent self explanatory. It will be observed that that difference between Figures 1 and 2 is that in Figure 2 there is provided a metallic tube 12 encircling the end turns of the heater 9 in the neighbourhood of the insulator 7, said tube being welded or otherwise mechanically and electrically united with the cap 8 and, therefore, with the metallic end plate 5 of the housing within which the heater is situated. The heater of Figure 2 is of rather smaller diameter than that of Figure 1 and the insulator 7 thereof is of slightly different shape from the insulator 7 of Figure l in order that the tube 12 may be properly located safely out of contact with the heater turns embraced thereby. The tube 12 is brought as close as is U practicable to the encircled turns of the heater 5 while avoiding risk of contact. The pressure of the tube 12 provides the required additional capacity.

In the modification shown, so far as is necessary to an understanding thereof, in Figure 3 a metallic skirt 13 is welded or otherwise mechanically and electrically united with the cathode tube 6, said skirt clearing the cap 8 round the insulator 7 and extending almost to the end disc 3. This skirt provides additional capacity between the disc 5 and the cathode tube 6.

In the further modification shown, so far as is necessary to an understanding thereof, in Figure 4, a disc like member 14, apertured to pass the heater end connection to the connection point 11 is fitted over the insulator 7 on the side thereof remote from the metallic end plate 5 and is arranged to be in electrical connection with the cathode tube 6 so as to provide additional capacity be tween said tube and the end plate 5.

We claim:

1. A magnetron cathode structure wherein a cathode heater is contained within a cathode-carrying box-like housing which is partially conductive but has a metallic end piece and an insulator insulating said end piece from the remaining metal parts of the housing, said magnetron cathode structure having means for providing additional capacity between said end piece and said remaining metal parts of said housing.

2. A magnetron cathode structure as claimed in claim 1 wherein the end turns of the heater adjacent an end insulator forming part of the housing is encircled by a metallic tube electrically and mechanically united with a metallic end plate of the housing to which said end of the heater is connected and which is insulated from the rest of the housing.

3. A magnetron cathode structure as claimed in claim 1 wherein a skirt member is electrically and mechanically connected to a cathode tube forming part of the housing near an end which incorporates an insulator, said skirt member extending over the insulator towards a metallic end plate of the housing on the other side of said insulator and to which the adjacent end of the heater is connected.

4. A magnetron cathode structure as claimed in claim 1 wherein the cathode structure comprises a cathode tube and wherein a metal disc-like member apertured "to pass the connection from the heater to the metallic end plate adjacent the end insulator, is placed over said insulator on the side thereof remote from said metallic end plate and is connected to the cathode tube.

5. A magnetron cathode structure as claimed in claim 1 wherein the additional capacity provided is at least 5 pico-farads.

References Cited in the file of this patent UNITED STATES PATENTS 2,414,605 Okress et al Ian. 21, 1947 2,510,267 Tolson June 5, 1950 2,569,847 Eitel et al Oct. 2, 1951 and 

